Memory units for binary signals are among the most important functional units of digital information processing systems. They are used for example as program and data memory units in digital computers, measuring devices, as code converters and generally for producing combinational logic circuits. A digital memory unit is an arrangement of a plurality of storage locations, which are organized so that every storage location can be accessed from outside. In what is known as a word-organized memory unit all the bits of a word can be addressed simultaneously and read or input in parallel. In the case of a k-bit word therefore, all the k bits of a word are considered to be a storage location. Unless-otherwise stated, a storage location below always refers to a k-bit long word. When only one bit is addressed at a time, it is referred to as bit organization.
The former application DE 100 37 992 describes an online test concept, with which hardware is tested in blocks at regular intervals during operation. Time intervals are predefined for this, in which the hardware is tested in cycles and then the original state is restored, before functional operation is continued. The described concept for example allows online testing of state machines (FSM=Finite State Machine) and what is known as random logic.